The high cost of energy has led to the search for economical processes for recovering and purifying alternate fuel sources such as landfill gas, biogass and synthesis gas. However, the high energy costs associated with removing CO.sub.2 from fuel sources such as methane, have hampered efforts to fully utilize alternate fuel sources and have restricted recovery efforts to larger fuel sources where economies of scale are more favorable.
Cyclic pressure swing adsorption systems are one of the technologies currently being employed to remove CO.sub.2 from the above-mentioned fuel sources. These systems are designed to fractionate gaseous mixtures by selective adsorption wherein the gaseous mixture is passed through a plurality of adsorption columns containing adsorbent beds which selectively retain the secondary component, in this case CO.sub.2, allowing for collection of the primary component, namely, the fuel source.
Representative of pressure swing adsorption processes is U.S. Pat. No. 4,077,779, issued to Air Products and Chemicals, Inc., wherein a gaseous mixture is separated into a primary gaseous product and a secondary gaseous product in a system comprising a plurality of adsorption zones operated in cycle in a predetermined timed sequence. Each adsorption zone contains a bed of solid sorbent preferentially selective toward the secondary gaseous product thereby allowing for separation and collection of the desired primary product. The method comprises the following sequence of operational steps performed in the order recited in each of the adsorption zones:
(a) passing the gaseous mixture to be separated through the bed of sorbent while discharging from the zone unsorbed gas as primary gaseous product, the adsorption zone having been previously brought to the desired superatmospheric feed gas pressure with primary gaseous product withdrawn from another adsorbent zone of the system;
(b) terminating the operation of step (a) as applied to the designated adsorbent zone and rinsing the zone in a direction concurrent to the initial feed with a stream of recompressed secondary gaseous product while discharging from the zone a gaseous rinse effluent which is recycled to another adsorbent zone then undergoing step (a) for admixture with gaseous feed mixture being admitted into that adsorption zone;
(c) lowering the pressure in the zone having undergone step (b) to an intermediate level to desorb residual gas and withdrawing such residual gas from the zone in a direction countercurrent to the feed gas flow, the withdrawn gas being recompressed for use, at least in part, to rinse an adsorption zone of the system undergoing step (b);
(d) purging the zone that has just undergone step (c) with air or an inert gas less strongly sorbed than the secondary gaseous component of the feed gas mixture;
(e) evacuating the purged zone to a sub-atmospheric pressure level to remove purge gas introduced in step (d); and
(f) repressurizing the zone to about the feed pressure of step (a) by admitting a part of the primary product gas component withdrawn from an adsorption zone then undergoing step (a).
U.S. Pat. No. Re. 31,014 (originally issued as U.S. Pat. No. 4,171,206), discloses a nine-bed pressure swing cyclic adsorption system for separating a multi-component feed gas into a primary product and a secondary product wherein an adsorbent bed within the column selectively retains the secondary product. Following the adsorption step, the secondary product is desorbed from the adsorption bed by high pressure rinsing and pressure equalization steps. More particularly, pure secondary product is admitted countercurrently through the adsorption bed to rinse void gas and desorbed gas into a second bed. The rinsed first bed is then connected to another adsorption bed which has previously been evacuated to the lowest pressure in the cycle causing pressure to equalize within the two adsorption beds.